The cellular requirements for the initiation of tolerance to a thymic dependent antigen will be evaluated by the utilization of cell transfer techniques. We also wish to determine if the ability of normal cell populations to synthesize antibody, to cooperate in an antibody response and to divide in response to antigen can be inhibited by cells from tolerant animals. Experiments will be aimed at determining the site of action of suppressor cells in the initiation of tolerance to human gamma globulin and at analyzing the role of immunopotentiators in the induction of tolerance to human gamma globulin. Furthermore, attempts will be made to activate bone marrow and thymus-derived lymphocytes from tolerant animals, and to subsequently establish whether or not immunologically inactive cell populations in tolerant animals are eliminated after tolerance induction or suppressed by a mechanism of active facilitation.